THERMOKINEMATIC MODELING OF SHORTENING IN THE TAJIK FOLD AND THRUST BELT: IMPLICATIONS FOR INTRACONTINENTAL SUBDUCTION BENEATH THE PAMIR MOUNTAINS

The Pamir Mountains are the archetype for active intracontinental subduction. A low-velocity zone and deep seismicity in the upper mantle beneath the Pamir have been widely interpreted as subducted Asian lithosphere. Proponents of the intracontinental subduction model have suggested that the Pamir has advanced ~300 km over its foreland, the Tajik-Tarim Basin, and that upper to mid-crustal rocks have been underplated or interleaved into the Pamir while the lower crust and mantle lithosphere is subducted beneath the Pamir. We present a regional balanced cross-section across the Tajik Fold and Thrust Belt (TFTB) and new apatite (U-Th)/He and apatite fission track data that is combined with thermokinematic numerical modeling to constrain the magnitude of shortening (~70 km) and timing of deformation (Miocene to present) in the TFTB. More than half of the shortening in the TFTB is related to underthrusting of Tajik-Tarim lithosphere beneath the Tian Shan Mountains, northwest of the Pamir, leaving only ~30 km of shortening or underthrusting beneath the Pamir. This amount of shortening is significantly less than predicted by models that call for subduction of an ~300 km long slab of continental Tajik-Tarim lithosphere beneath the Pamir. Furthermore, a relatively complete stratigraphic sequence from Paleozoic metamorphic basement in the Northern Pamir to Neogene synorogenic deposits in the Tajik Basin are present along the margin of the Pamir and a small klippe of Tajik Basin sediments in preserved in the interior of the Pamir, suggesting that the Tajik-Tarim lithosphere was not subducted, but uplifted and eroded during growth and expansion of the Pamir orogen. We suggest that the low-velocity zone and south-dipping zone of deep seismicity beneath the Pamir is related to gravitational foundering (by delamination or large-scale dripping) of eclogitized lower crust and lithospheric mantle of the thickened Pamir lithosphere, rather than cratonic Asia.